The present invention is for a catalyst for use in the Fischer-Tropsch process, and for a method to prepare the catalyst. The catalyst of the present invention has a higher surface area, more uniform metal distribution, and smaller metal crystallite size than Fischer-Tropsch catalysts of the prior art.
Since about 1923, the Fischer-Tropsch (F-T) process, which involves passing a continuous stream of synthesis gas (“syngas” or a mixture of hydrogen gas and carbon monoxide) over a supported metal catalyst, has been used for the conversion of syngas into higher value commercial products, such as gasoline, diesel fuels, linear alcohols, and α-olefins. The catalysts used in the process are typically in the form of a pellet or powder having metal active sites on the surface of an essentially chemically inert material carrier. When the syngas reactants contact an active site on the catalyst, the carbon monoxide bonds are cleaved and hydrogenated producing a mixture of hydrocarbon products. For commercial operations, it is desirable that the gas be passed over the catalyst at an essentially constant and rapid rate. But because an active site can only be occupied by one molecule at a time, the most effective catalysts have a large number of active sites and high turnover (or conversion) rate.
Like all catalysts, the F-T catalyst itself is not permanently altered in the reaction. However, over time catalyst efficiency can be diminished by contamination of the active sites, for example, by deposition of carbon or other contaminants in the syngas feed or by coking or by the deposition of waxy hydrocarbon on the catalyst surface, thus requiring that the catalyst bed be cleaned or regenerated. Further, the catalyst efficiency can be irreversibly diminished if the catalyst particles sinter (fuse together) or crumble while the catalyst is packed in the catalyst bed because the syngas flow is then restricted and the number of available active sites are decreased. Because most commercial operations use continuous syngas streams, it can be costly and inconvenient to clean or otherwise regenerate or replace the catalyst bed. Thus, the most desirable Fischer-Tropsch catalysts can be used for an extended period between catalyst regeneration and do not require frequent replacement of the catalyst bed when exposed to normal industrial processing conditions.
As is known in the art, the composition and physical characteristics of the Fischer-Tropsch catalyst particles affect the catalyst activity. Typically, F-T catalysts include one or more metals selected from Group VIII of the Periodic Table of Elements (iron, cobalt, nickel, ruthenium, rhenium, palladium, osmium, iridium, platinum), a promoter, and a carrier or support. The Group VIII metal is added to effect the conversion of the syngas, and is selected based on the feed composition and the desired product mixture. (For a more extensive discussion of the F-T process see, for example, “Practical And Theoretical Aspects Of The Catalytic Fischer-Tropsch Process,” Applied Catalysis A: General 138 (1996) 319-344 by M. E. Dry, incorporated herein by reference.) Cobalt is commonly used in F-T catalysts because of its commercial availability, its efficiency in converting the syngas to longer chain hydrocarbons, its ease of handling, its low activity in water-gas shift reactions, and its relative low cost as compared to other Group VIII metals. The promoters are added to improve certain properties of the catalyst or to improve the catalyst selectivity, and ruthenium, copper, and alkali metals are commonly used promoters for cobalt-based catalysts. The carriers, such as silica, alumina, or alumino-silicates, provide a means for increasing the surface area of the catalyst. For a more extensive review of Fischer-Tropsch catalyst compositions, see for example, U.S. Pat. No. 5,248,701, issued to Soled et al, and the references therein (incorporated herein by reference).
The physical characteristics of the Fischer-Tropsch catalyst are also important. Because the hydrogen gas and carbon monoxide must make physical contact with the Group VIII metal for the conversion to occur, catalyst particles with uniform metal distribution, homogeneous metal loading and high surface areas have higher activity rates in a commercial scale slurry bed reactor than particles with the metal localized on the surface.
Thus, it would be beneficial to have a cobalt-based Fischer-Tropsch catalyst that has a high surface area, a smooth, homogeneous surface morphology, and a uniform distribution of metal throughout the catalyst. Because studies have shown that the metal crystallite size might affect the hydrogenation reactions, the catalyst would preferably have a smaller crystallite size than current Fischer-Tropsch catalysts. Further, the catalyst should be easy to prepare on a commercial scale.